Jökull - 01.12.1989, Side 70

Similarity coeflicient
Table III. Simple regression results for shape
parameters and grain size. The values t and P show
the results of t-tests for the statistical significance of
the correlation coefficient, r, for each pair of vari-
ables; n, number of samples (grains in case of size
tests).
Tafla III. Tölfrœðilegar niðurstöður varðandi form-
þœtti og kornastœrð. Gildin t ogP sýna niðurstöður
t-prófs á marktœkni fylgnistuðulsins r milli hvers
hreytupars; n táknar fjölda sýna (korna, þegar um
er að rœða prófun á stœrð).
Dependent variable Independent variable n r t P
Elongation Si02 19 0.564 -2.819 0.012
Sphericity Si02 19 0.463 -2.154 0.046
OP-index Si02 19 0.490 2.318 0.033
Elong. var. Si02 19 0.616 3.227 0.005
Elongation Sphericity 22 0.899 9.165 <0.000
Elongation OP-index 22 0.972 -18.381 <0.000
Elongation Size 1760 0.059 -2.463 0.014
Sphericity Size 1760 0.129 -5.466 <0.000
OP-index Size 1760 0.005 -0.189 0.850
up with the sample from Karl (submarine phreato-
magmatic). Although all the samples from phreato-
magmatic eruptions (Surtsey, Katla, Karl) form part
of the same cluster, it is quite clear that this cluster
diagram joins samples from dissimilar eruptions in
several cases. The cluster diagram in Fig. 6 should
be compared with the Sneed-Folk diagram in Fig 2,
where nearly all the samples plot in only two of the
form cells (compact bladed and bladed).
The second cluster diagram (Fig. 7) is based on
all the parameters used in Fig. 6 as well as the calcu-
lated form parameters, their variance, and the Si02
percentage. The data were standardized (Wilkinson,
1987) before clustering. The topmost cluster is
formed by samples from Öræfajökull 1362 (plinian),
Skógar (subglacial phreatomagmatic or plinian?),
Kúðafljót (unknown eruption) and H5 (light
coloured, plinian phase). Again, the Surtsey, Katla,
and H4 - final phase samples are clustered, but the
Heimaey sample joins later after the incorporation
of H5 (dark, andesitic part), and the H4-initial phase
1.000 0.500
1---------------1----------------1----------------1---------------1----------------f
Eruption Sample
Fig. 6. Cluster diagram for samples, based on the
parameters C, CP, CB, CE, P, B, E, VP, VB, and VE,
which represent the number of plotted points in
respective cells of the Sneed-Folk diagram. The
similarity scale at the top of the diagram is the Pear-
son correlation coefficient.
Mynd 6. Línurit yfir fylgni milli sýna, þegar byggt
er á fjölda korna, sem raðast í formflokkana C, CP,
CB, CE, P, B, E, VP, VB og VE á Sneed - Folk grafi,
sjá nrynd 2. Skyld sýni raðast saman í kippur vinstra
megin á línuritinu. Kvarðinn efst á línuritinu er
fylgnistuðull Pearsons.
- H^ pair in a larger cluster.
The third cluster diagram (Fig. 8) leaves out the
Sneed-Folk cell frequency parameters and the Si02
percentage. On this diagram, the Kúðafljót sample
(unknown eruption site), which stands alone on the
first one, is joined to a cluster formed by Öræfajök-
ull (plinian), H5 (plinian), Skógar (unknown erup-
tion site), and H^. Surtsey and H4 - final phase con-
tinue to join in a cluster which is highly correlated
to a cluster formed by Katla, H5, H1693, and Ht.
It is obvious from the dendrograms in Figs. 6-8
that the clustering is quite sensitive to the selection
of parameters. Looking at the similarity coefficient
68 JÖKULL, No. 39, 1989